Linear actuators are electromechanical devices which provide linear mechanical motion in response to an electrical input. In many applications, the magnitude of the force applied by the actuator need not be constant over the full length of the actuator stroke. By way of example, an actuator for the marking pen of an X-Y plotter device must transfer the pen from a home position to the location to be marked. Movement from the home position to the marking position requires a relatively small amount of force and a relatively long translation. Once the pen is in position, the actuator forces the pen against the printing medium, such as paper. In this portion of the stroke, a relatively large amount of force is required to firmly press the pen tip against the medium so that marking is accomplished.
Heretofore, linear actuators have been designed so as to apply a relatively uniform force over the full stroke length for a constant input current. It would be desirable to provide an actuator which is capable of applying a varying force over the length of the stroke in those applications where a constant force is not necessary or not desired.
The present invention is directed to a linear actuator which provides a non-uniform, but controllable force along the length of the stroke. One advantage is that lower performance and, hence, less costly, magnetic material can be used in those locations where a reduced force is adequate. More costly and higher performance magnetic material need be used only in those locations where greater actuating force is necessary.
A further advantage of utilizing high performance magnetic material only where required is that the total flux in the soft iron portions of the magnetic circuit is reduced. As a result, the dimensions of the soft iron members can be reduced without encountering magnetic saturation of the members. Thus, cost is further reduced as is the weight of the actuator. These and other advantages of the present invention will be apparent to those skilled in the art upon a reading of the following Best Mode for Carrying Out the Invention together with drawings.